Use RuboCop Performance 1.21

This commit disables `Performance/StringIdentifierArgument` under the `test/**` directory
to suppress the following detection:

```console
test/rubocop/cop/minitest/assert_match_test.rb:7:19: C: [Correctable] Performance/StringIdentifierArgument:
Use :"test_registers_offense_when_using_assert_with_#{matcher}" instead of "test_registers_offense_when_using_assert_with_#{matcher}".
    define_method("test_registers_offense_when_using_assert_with_#{matcher}") do
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

This setting is because the code in this directory is test code, and performance is not of utmost importance.
Additionally, `:"Use ..."` is not necessarily more readable than `"Use ..."`.

.rubocop.yml

@@ -117,3 +117,7 @@ Naming/FileName:
 Metrics/ModuleLength:
   Exclude:
     - 'test/**/*.rb'
+
+Performance/StringIdentifierArgument:
+  Exclude:
+    - 'test/**/*.rb'

Gemfile

@@ -12,7 +12,7 @@ gem 'minitest-proveit'
 gem 'prism'
 gem 'rake'
 gem 'rubocop', github: 'rubocop/rubocop'
-gem 'rubocop-performance', '~> 1.18.0'
+gem 'rubocop-performance', '~> 1.21.0'
 gem 'test-queue'
 gem 'yard', '~> 0.9'